Abstract

Objectives: The authors examined the increase in mortality associated with hot and cold temperature in different locations, the determinants
of the variability in effect estimates, and its implications for adaptation.

Methods: The authors conducted a case-crossover study in 50 US cities. They used daily mortality and weather data for 6 513 330 deaths
occurring during 1989–2000. Exposure was assessed using two approaches. First, the authors determined exposure to extreme
temperatures using city-specific indicator variables based on the local temperature distribution. Secondly, they used piecewise
linear variables to assess exposure to temperature on a continuous scale above/below a threshold. Effects of hot and cold
temperature were examined in season-specific models. In a meta-analysis of the city-specific results, the authors examined
several city characteristics as effect modifiers.

Results: Mortality increases associated with both extreme cold (2-day cumulative increase 1.59% (95% CI 0.56 to 2.63)) and extreme
heat (5.74% (95% CI 3.38 to 8.15)) were found, the former being especially marked for myocardial infarction and cardiac arrest
deaths. The increase in mortality was less marked at less extreme temperatures. The effect of extreme cold (defined as a percentile)
was homogeneous across cities with different climates, suggesting that only the unusualness of the cold temperature (and not
its absolute value) had a substantial impact on mortality (that is, acclimatisation to cold). Conversely, heat effects were
quite heterogeneous, with the largest effects observed in cities with milder summers, less air conditioning and higher population
density. Adjustment for ozone led to similar results, but some residual confounding could be present due to other uncontrolled
pollutants.

Conclusions: The authors confirmed in a large sample of cities that both cold and hot temperatures increase mortality risk. These findings
suggest that increases in heat-related mortality due to global warming are unlikely to be compensated for by decreases in
cold-related mortality and that population acclimatisation to heat is still incomplete.

Footnotes

Funding: This study was funded by EPA Grant R832416-010 and NIEHS grant ES-0002. None of the funding institutions was involved
in the study design, collection, analysis or interpretation of data, or in the writing of the report or submission for publication.